The present invention relates to radio communications, and in particular, to balancing cell loads in a cellular radio communications network.
In a cellular communications system, a mobile radio station communicates over an assigned radio channel with one or more radio base stations. Several base stations are connected to a switching node which is typically connected to a gateway that interfaces the cellular communications system with other communication systems. A call placed from an external network to a mobile station is directed to the gateway, and from the gateway through one or more switching nodes to one or more base stations serving the called mobile station. The base station(s) pages the called mobile station and establishes a radio communications channel. A call originated by the mobile station follows a similar path in the opposite direction.
In a Code Division Multiple Access (CDMA) mobile communication system, spreading codes are used to distinguish information associated with different mobile stations or base stations transmitting over the same radio frequency band. In other words, individual radio xe2x80x9cchannelsxe2x80x9d correspond to and are discriminated on the basis of these codes. Various aspects of CDMA are set forth in one or more textbooks such as Applications of CDMA and Wireless/Personal Communications, Garg, Vijay K. et al., Prentice-Hall 1997.
Spread spectrum communications permit mobile transmissions to be received at two or more (xe2x80x9cdiversexe2x80x9d) base stations and processed simultaneously to generate one received signal. With these combined signal processing capabilities, it is possible to perform a handover from one base station to another, (or from one antenna sector to another antenna sector connected to the same base station), without any perceptible disturbance in the voice or data communications. This kind of handover is typically called soft or diversity handover.
During diversity handover, the signaling and voice information from plural sources is combined in a common point with decisions made on the xe2x80x9cqualityxe2x80x9d of the received data. In soft handover, as a mobile station involved in a call moves to the edge of a base station""s cell, the adjacent cell""s base station assigns a transceiver to the same call while a transceiver in the current base station continues to handle that call as well. As a result, the call is handed over on a make-before-break basis. Soft handover is therefore a process where two or more base stations handle the call simultaneously until the mobile station moves sufficiently close to one of the base stations which then exclusively handles the call. xe2x80x9cSofterxe2x80x9d handover occurs when the mobile station is in handover between two different antenna sectors connected to the same, multi-sectored base station using a similar make-before-break methodology.
Certain problems must be considered in a CDMA communications system. Because all users transmit information using the same frequency band at the same time, each user""s communication interferes with the communications of the other users. Therefore, the power of radio transmitters in a CDMA system must be carefully controlled. Another problem is that the physical characteristics of a radio channel vary significantly. For example, the signal propagation loss between a radio transmitter and receiver varies as a function of their respective locations, obstacles, weather, etc. As a result, large differences may arise in the strength of signals received at a radio receiver from different radio transmitters. If the transmission power of a radio transmitter signal is too low, the receiver may not correctly decode a weak signal, and the signal will have to be corrected (if possible) or retransmitted. Accordingly, erroneous receipt of signals adds to congestion in a cell. Accordingly, a base station allocates desired transmit powers to downlink traffic channels, (i.e., base-to-mobile stations), so that the mobile stations receive the traffic information at an appropriate signal level. The transmit power allocated to such traffic channels may be adjusted to accommodate changing channel conditions resulting from movements of mobile stations, multipath propagation, weather, obstacles, and current interference level experienced in a cell. But the problem with increasing transmissions of one communication is that it adversely impacts other communications in the same cell or even adjacent cells by increasing the interference level for those other communications. Thus, the transmit power levels corresponding to those other communications may also be increased in response to the increased interference which further compounds the overall interference problem. When the traffic load in a particular cell among the plurality of cells in a mobile communications network exceeds an overload condition, (i.e., its capacity of traffic channels, traffic channel power, etc.), that cell is forced to block new mobile radio calls, or to even drop existing calls, in particularly severe overload conditions. In both cases, the system performance capacity is adversely impacted.
There is often a situation where two neighboring cells may have different traffic loads. The traffic load in one cell is too high, and the traffic load in a neighboring cell is substantially lower. Rather than have the overloaded cell reject calls or suffer deteriorated performance for existing calls, it would be desirable to move some of the traffic from the overloaded cell to the non-overloaded cell to balance the transmit power level in those cells. By transferring some of the traffic load from the high load cell, the transmit power level in that overloaded cell is decreased, which improves performance in the loaded cell. Performance is also improved in the underloaded cell because the transferred traffic means that the resources in the underloaded cell are more efficiently utilized. In addition, the service provided to the transferred mobile users is not interrupted, and newly requested services need not be denied.
One way of transferring traffic load from an overloaded cell is to lower the handover threshold in that cell. FIG. 1 shows three neighboring cells A-C. Cell A contains radio user equipment nodes A and B (UE-A and UE-B) and is currently in an overloaded condition. Cells B and C are not overloaded, and therefore have additional capacity. By lowering the handover threshold for overloaded cell A, the cell border of cell A is effectively reduced from the normal cell border to a smaller, xe2x80x9cshedxe2x80x9d cell border. Because user equipment A is outside the shed cell border, UE-A is handed over to one of cells B and C. By shedding user equipment connections from overloaded cell A to an underloaded cell (B or C), the traffic load between these three cells is balanced.
However, there are problems with changing the handover threshold in overloaded cell A to effect handover of UE-A to cell B or C. First, the uplink transmit power required (from UE to base station) to transmit signals from UE-A to the new cell B or C must increase in order to reach the base station in cell B or C. In other words, UE-A has a shorter distance to transmit in the uplink direction to the base station in cell A than it does to the base station in cell B for example. Having been handed over to cell B, UE-A must increase its transmit power to reach the base station in cell B. Uplink interference is increased as a result of the increased power of transmission in the uplink direction by UE-A. The increased interference causes other radio transmitters in cell A, B and C to increase their transmission power to compensate for the increased interference. The increases in transmission power by UE-A and these other radio transmitters further increase interference in surrounding cells. Second, because UE-A is required to increase its uplink transmit power, it consumes more battery power than if it were simply transmitting to closer cell A. Third, having to perform a handover of the connection from cell A to cell B requires added control signaling, and there is always a risk the connection may be lost.
It is an object of the present invention to provide a transmit power balancing scheme that optimizes the capacity of a mobile radio communications system.
It is a further object of the present invention to provide downlink transmit power balancing without increasing uplink interference.
It is still a further object to provide load balancing between overloaded and underloaded cells that does not require handover and handover control signaling.
The present invention solves the above-identified problems and meets these and other objectives using a transmit power balancing method that increases overall communication capacity without incurring substantial, additional control signaling. An overload condition in a first cell serviced by a radio network is detected, and a second nearby cell which is not overloaded is identified. For a radio user node that has a connection with the radio network, radio transmission from the first cell is prevented or at least avoided in a downlink direction from the radio network to the radio user node. Instead, a radio transmission associated with that connection is established or otherwise permitted from the second cell in the downlink direction to the radio user node. In a preferred example embodiment, the prevented or avoided downlink radio transmission is a traffic transmission. On the other hand, downlink control signaling associated with the connection from the first cell to the radio user node is permitted. Also, in the preferred example embodiment, radio transmissions from the radio user node to the first cell in the uplink direction are permitted, and preferably maintained, while transmissions in the downlink direction originate from the second cell.
In a more detailed, non-limiting example implementation of the invention, the radio network broadcasts to radio user nodes in or near the first cell an xe2x80x9cavoid cellxe2x80x9d message instructing radio user nodes either to avoid using or to not use the first cell for downlink transmissions. In response, a radio user node in the first cell can transmit an indicator to the radio network that downlink transmissions associated with the connection to the radio user node should preferably or necessarily be from the second cell depending on how the invention is implemented. In an implementation where the first cell is to be avoided, a scaling factor may be used to reduce the chances that the first cell is selected.